Semiconductor device including voltage level conversion output circuit

ABSTRACT

A semiconductor device for easily changing an operating voltage of an I/O circuit. The I/O circuit includes a first I/O cell, which operates with a first high-potential power supply, and a second I/O cell, which operates with a second high-potential power supply. The I/O circuit includes a control circuit for selectively activating the first and second I/O cells according to a voltage selection signal. In the I/O circuit, a signal having a voltage according to an operating voltage of the selected I/O cell is generated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-238326, filed on Aug. 18,2004, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device, and moreparticularly, to a semiconductor device including an output circuit thatconverts an output signal level of an internal circuit to a levelaccording to an externally connected device.

In recent years, semiconductor devices are increasingly highlyintegrated, and operate at higher speed. To reduce power consumption,efforts have also been made to lower the driving voltage ofsemiconductor devices. However, some semiconductor devices, whosedriving voltage is yet to be lowered, still operate at high drivingvoltages. A semiconductor device may be connected to a plurality ofother semiconductor devices that have different power supply voltages.Such a semiconductor device includes input and output (I/O) circuits forgenerating operating voltages corresponding to the power supply voltagesof the other semiconductor devices.

The I/O circuits are conventionally arranged on the periphery of thesemiconductor device. The other semiconductor devices and such elementsas resistors are connected to the I/O circuits. To distinguish betweensemiconductor devices, the semiconductor device on which attention isfocused is herein referred to as a “main semiconductor device”, and asemiconductor device connected to the main semiconductor device isherein referred to as a “sub-semiconductor device”.

The operating voltage of an I/O circuit of a sub-semiconductor devicediffers depending on its type (e.g., its memory). The main semiconductordevice includes a plurality of I/O circuit blocks corresponding tooperating voltages of one or more sub-semiconductor devices. Each I/Ocircuit block provides the corresponding operating voltage via aterminal (pad).

For example, as shown in FIG. 1, I/O blocks 12 a, 12 b, 12 c, and 12 dare arranged on the periphery of a main semiconductor device 11, and aninternal circuit 13 is arranged inside the blocks 12 a, 12 b, 12 c, and12 d. Each of the blocks 12 a, 12 b, 12 c, and 12 d includes a pluralityof I/O circuits. Each block may include input circuits or outputcircuits instead of I/O circuits.

The blocks 12 a to 12 d are connected to a correspondingsub-semiconductor device. Each of the blocks 12 a to 12 d receives apower supply voltage according to the power supply voltage of thecorresponding sub-semiconductor device, and generates a signal having alevel corresponding to the received power supply voltage.

For example, each of the first and third blocks 12 a and 12 c isconnected to a sub-semiconductor device that includes an I/O circuit,which operates at a first power supply voltage VDD1 (e.g., 1.8 V). Eachof the blocks 12 a and 12 c is supplied with the first power supplyvoltage VDD1. Also, each of the second and fourth blocks 12 b and 12 dis connected to a sub-semiconductor device that includes an I/O circuit,which operates at a second power supply voltage VDD2 (e.g., 3.3 V). Eachof the blocks 12 b and 12 d is supplied with the second power supplyvoltage VDD2.

The internal circuit 13 operates at a predetermined internal powersupply voltage VDDI (e.g., 1.2 V), and inputs and outputs a signalhaving a level corresponding to the internal power supply voltage VDDI.Thus, each I/O circuit included in the first and third blocks 12 a and12 c includes a level conversion circuit for converting voltages betweenthe first power supply voltage VDD1 and the internal power supplyvoltage VDDI. Each I/O circuit included in the second and fourth blocks12 b and 12 d includes a level conversion circuit for convertingvoltages between the second power supply voltage VDD2 and the internalpower supply voltage VDDI.

Each of the first and third blocks 12 a and 12 c converts the voltage ofa signal from the internal circuit 13 of the main semiconductor deviceinto the first power supply voltage VDD1, and provides the correspondingsub-semiconductor device with a signal having the first power supplyvoltage VDD1. Each of the second and fourth blocks 12 b and 12 dconverts the voltage of a signal from the internal circuit 13 into thesecond power supply voltage VDD2, and provides the correspondingsub-semiconductor device with a signal having the second power supplyvoltage VDD2.

Pads of each of the blocks 12 a to 12 d are connected to power supplywiring to which the first power supply voltage VDD1 or the second powersupply voltage VDD2 is provided. With this structure, the operatingvoltage of each of the blocks 12 a to 12 d is changed in correspondencewith the operating voltage of the connected sub-semiconductor device, bychanging the power supply voltage provided to the power supply wiring ofeach of the blocks 12 a to 12 d.

SUMMARY OF THE INVENTION

The operating voltage of only some of the I/O circuits included in oneblock (i.e., some of the I/O circuits included in the block 12 a) mayrequire changing in correspondence with the connected sub-semiconductordevice. However, a plurality of I/O circuits included in each of theblocks 12 a to 12 d are commonly connected to the same power supplywiring. It is impossible to change the operating voltage of only someI/O circuits included in one block. To enable the operating voltage ofonly some I/O circuits to be changed, a mask used in manufacturingprocesses for the main semiconductor device needs to be newly created.Creating a new mask requires a great number of days, and increases themanufacturing cost of the main semiconductor device.

Further, in the main semiconductor device 11 including the I/O blocks 12a to 12 d, pads relating to each of power supply wiring groups need tobe arranged close to one another. This decreases flexibility inarranging pads.

To increase flexibility in arranging pads, the number of power supplywiring groups may be increased. However, each power supply wiring groupmust be supplied with the required power supply. This means that anincrease in the number of power supply wiring groups increases thenumber of required power supplies (the number of power supply wiringsand the number of pads for providing power supplies). Further, eachpower supply wiring group needs to be isolated from one another by awell. Thus, the area of wells decreases, and the electrostatic discharge(ESD) withstand voltage of the main semiconductor device 11 decreases.

The present invention provides a semiconductor device that easilychanges the operating voltage of an I/O circuit.

One aspect of the present invention is a semiconductor device includingan internal circuit for generating an output signal, and an outputcircuit, connected to the internal circuit, for converting a voltagelevel of the output signal of the internal circuit. The output circuitincludes a plurality of output cells for generating a plurality oflevel-converted output signals having different voltages, respectively.A control circuit, connected to the plurality of output cells, selectsone of the plurality of output cells according to a voltage selectionsignal.

Another aspect of the present invention is a semiconductor deviceincluding an internal circuit for generating an output signal, and anoutput circuit, connected to the internal circuit, for converting avoltage level of the output signal of the internal circuit. The outputcircuit includes a plurality of first output transistors connected to aplurality of high-potential power supplies and having different voltage,respectively. A second output transistor is connected to a low-potentialpower supply and to the plurality of first output transistors. A controlcircuit selects one of the plurality of first output transistorsaccording to a voltage selection signal and causes the selected firstoutput transistor and the second output transistor to perform a levelconversion operation.

A further aspect of the present invention is a semiconductor deviceincluding an internal circuit for generating an output signal. An outputcircuit, connected to the internal circuit, converts a voltage level ofthe output signal of the internal circuit. The output circuit includes aplurality of first output transistors connected to a plurality ofhigh-potential power supplies having different voltages, respectively. Asecond output transistor is connected to a low-potential power supplyand to the plurality of first output transistors. A control circuitselects, in a first mode, one of the plurality of first outputtransistors according to a voltage selection signal and causes theselected first output transistor and the second output transistor toperform a level conversion operation, and sequentially controls, in asecond mode, the plurality of first output transistors according to thevoltage of each of the plurality of high-potential power supplies.

Another aspect of the present invention is a semiconductor deviceincluding an internal circuit for generating an output signal. An outputcircuit, connected to the internal circuit, converts a voltage level ofthe output signal of the internal circuit. The output circuit includes aplurality of output cells generating a plurality of level-convertedoutput signals having different voltages. A plurality of pads arerespectively connected to the plurality of output cells.

Other aspects and advantages of the present invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a conventional semiconductor device;

FIG. 2 is a schematic diagram of a semiconductor device of the presentinvention;

FIG. 3 is a schematic block diagram of an I/O block included in thesemiconductor device of FIG. 2;

FIG. 4 is a circuit diagram of an I/O circuit included in the I/O blockof FIG. 3 according to a first embodiment of the present invention;

FIG. 5 is a schematic diagram showing a part of the semiconductor deviceof FIG. 2;

FIG. 6 is a circuit diagram of an I/O circuit included in the I/O blockof FIG. 3 according to a second embodiment of the present invention;

FIG. 7 is a schematic layout diagram of the I/O circuits of FIG. 6;

FIG. 8 is a circuit diagram of an I/O circuit included in the I/O blockof FIG. 3 according to a third embodiment of the present invention;

FIG. 9 is an operation waveform diagram of the I/O circuit of FIG. 8;

FIG. 10 is a schematic layout diagram of I/O circuits according to afirst modification;

FIG. 11 is a schematic layout diagram of output transistors included inan I/O circuit according to a second modification;

FIG. 12(a) is a circuit diagram of an I/O circuit according to a thirdmodification, and FIG. 12(b) is a circuit diagram showing a modifiedexample of a part of the I/O circuit of FIG. 11;

FIG. 13 is a schematic block diagram showing a part of a semiconductordevice according to a fourth modification;

FIG. 14 is a schematic block diagram showing a first modified example ofa part of the semiconductor device of FIG. 13;

FIG. 15(a) is a schematic block diagram showing a second modifiedexample of a part of the semiconductor device of FIG. 13, and FIG. 15(b)is a schematic block diagram showing a third modified example of a partof the semiconductor device of FIG. 13;

FIG. 16(a) is a circuit diagram of an I/O circuit according to a fifthmodification, and FIG. 16(b) is a circuit diagram showing a modifiedexample of a part of the I/O circuit of FIG. 16(a);

FIG. 17 is a circuit diagram of an I/O circuit according to a sixthmodification;

FIG. 18 is a circuit diagram of an I/O circuit according to a seventhmodification;

FIG. 19 is a diagram showing determination results in the I/O circuit ofFIG. 18;

FIG. 20 is a schematic circuit diagram of a selection signal generationcircuit according to an eighth modification;

FIG. 21 is a circuit diagram of an I/O circuit according to a ninthmodification;

FIG. 22 is an operation waveform diagram of the I/O circuit of FIG. 21;

FIG. 23 is a circuit diagram of an I/O circuit according to a tenthmodification; and

FIG. 24 is an operation waveform diagram of the I/O circuit of FIG. 23.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, like numerals are used for like elements throughout.

The following describes a semiconductor device 21 according to a firstembodiment of the present invention, with reference to FIGS. 2 to 5.

A plurality of (eight in FIG. 2) input and output (I/O) blocks 22 a to22 h are formed on the periphery of the semiconductor device 21. Aninternal circuit 23 having various functions is formed in the center ofthe semiconductor device 21.

Each of the I/O blocks 22 a to 22 h includes a plurality of input andoutput (I/O) circuits 25 (or output circuits and/or input circuits)described later. The internal circuit 23 transmits and receives, via theI/O circuits 25, signals to and from other semiconductor devicesconnected to the semiconductor device 21. The internal circuit 23operates at an internal operating power supply VDDI, and inputs andoutputs signals to and from such elements as resistors connected to thesemiconductor device 21.

Power supply wirings 24 a, 24 b, and 24 c, which are formed as rings,are arranged on the I/O blocks 22 a to 22 h. The first power supplywiring 24 a is provided with a first high-potential power supply VDD1.The second power supply wiring 24 b is provided with a secondhigh-potential power supply VDD2. The third power supply wiring 24 c isprovided with a low-potential power supply VSS. The first and secondhigh-potential power supplies VDD1 and VDD2 are set at voltagesaccording to the operating power supply voltages of the othersemiconductor devices connected to the semiconductor device 21(specifically, the operating voltages of their input circuits) and tovoltages provided to the elements. For example, the first high-potentialpower supply VDD1 is set at 1.8 V, the second high-potential powersupply VDD2 at 3.3 V, and the low-potential power supply voltage at 0 V.The internal operating power supply VDDI provided to the internalcircuit 23 is set according to the operation of the internal circuit 23,and is set at a value substantially the same as the first high-potentialpower supply VDD1 in the present embodiment.

At least one of the I/O blocks 22 a to 22 h, namely, the I/O block 22 ain the present embodiment, is provided with the first and secondhigh-potential power supplies VDD1 and VDD2. Each of the other I/Oblocks 22 b to 22 h is provided with the first high-potential powersupply VDD1 or the second high-potential power supply VDD2 incorrespondence with the semiconductor device connected thereto. Forexample, each of the I/O blocks 22 b, 22 c, and 22 h receives the firsthigh-potential power supply VDD1, and inputs or outputs a signal havingthe level of the first high-potential power supply VDD1. Each of the I/Oblocks 22 d, 22 e, 22 f, 22 g, and 22 h receives the secondhigh-potential power supply VDD2, and inputs or outputs a signal havingthe level of the second high-potential power supply VDD2.

As shown in FIG. 3, the I/O block 22 a includes a plurality of (five inthe present embodiment) I/O circuits 25 a to 25 e. Each of the I/Ocircuits 25 a to 25 e is provided with the first and secondhigh-potential power supplies VDD1 and VDD2 and the low-potential powersupply VSS.

The I/O circuits 25 a to 25 e respectively receive, from the internalcircuit 23, signals DO0 to DO4 having a level according to the operatingvoltage of the internal circuit 23 (the first high-potential powersupply VDD1). The I/O circuits 25 a to 25 e respectively provide theinternal circuit 23 with signals DI0 to DI4 having a level according tothe operating voltage of the internal circuit 23.

The I/O circuits 25 a to 25 e are connected to pads 26 a to 26 e,respectively. The I/O circuit 25 a is provided with a voltage selectionsignal VSEL0. The I/O circuit 25 b is provided with a voltage selectionsignal VSEL1. The I/O circuits 25 c and 25 d are provided with a voltageselection signal VSEL2. The I/O circuit 25 e is provided with a voltageselection signal VSEL3. In the present embodiment, an external deviceprovides the voltage selection signals VSEL0 to VSEL3 to thesemiconductor device 21 via pads 27 a to 27 d included in thesemiconductor device 21.

The I/O circuits 25 a to 25 e each input and output signals having thelevel of the first high-potential power supply VDD1 or the level of thesecond high-potential power supply VDD2 according to the correspondingvoltage selection signals VSEL0 to VSEL3, to the correspondingsemiconductor devices via the corresponding pads 26 a to 26 e. Forexample, the I/O circuits 25 a to 25 e each input or output signalshaving the level of the first high-potential power supply VDD1 inresponse to the corresponding voltage selection signals VSEL0 to VSEL3at a low (L) level, and input or output signals having the level of thesecond high-potential power supply VDD2 in response to the correspondingvoltage selection signals VSEL0 to VSEL3 at a high (H) level.

Thus, appropriately setting the level of each of the voltage selectionsignals VSEL0 to VSEL3 enables all the I/O circuits 25 a to 25 eincluded in the I/O block 22 a to operate using the first high-potentialpower supply VDD1 or the second high-potential power supply VDD2 astheir operating voltage. Also, appropriately setting the level of eachof the voltage selection signals VSEL0 to VSEL3 enables only some of theI/O circuits 25 a to 25 e included in the I/O block 22 a to operateusing the first high-potential power supply VDD1 or the secondhigh-potential power supply VDD2 as their operating voltage. Further,the number of the I/O circuits whose operating voltage is changed isfreely set.

FIG. 4 is a circuit diagram of the I/O circuit 25 a.

The I/O circuit 25 a includes first and second I/O cells 31 a and 31 b,which are connected to the pad 26 a. The first I/O cell 31 a inputs andoutputs at the first high-potential power supply VDD1. The second I/Ocell 31 b inputs and outputs at the second high-potential power supplyVDD2. The I/O circuit 25 a is provided with a voltage selection signalVSEL0 and a control signal CIO. The control signal CIO is provided fromthe internal circuit 23, and controls whether to activate the first I/Ocell 31 a and the second I/O cell 31 b as an output cell. According tothe voltage selection signal VSEL0, one of the first and second I/Ocells 31 a and 31 b is activated to input or output a signal.

In detail, the I/O circuit 25 a includes two logic circuits 32 and 33,each of which receives a voltage selection signal VSEL0 and a controlsignal CIO. The first logic circuit 32 generates a signal S01 havingsubstantially the same level as the level of a control signal CIO inresponse to an L level voltage selection signal VSEL0, and generates anH level signal S01 in response to an H level voltage selection signalVSEL0. The second logic circuit 33 generates a signal S02 havingsubstantially the same level as the level of a control signal CIO inresponse to an H level voltage selection signal VSEL0, and generates anH level signal S02 in response to an L level voltage selection signalVSEL0.

The first I/O cell 31 a operates as an output cell in response to an Llevel signal S01, and converts a signal DO0 into a signal OUTa havingthe level of the first high-potential power supply VDD1. The second I/Ocell 31 b operates as an output cell in response to an L level signalS02, and converts a signal DO0 into a signal OUTb having the level ofthe second high-potential power supply VDD2.

The first I/O cell 31 a operates as an input cell in response to an Hlevel signal S01, and converts a signal INa, which is provided from anexternal semiconductor device via the pad 26 a, into a signal DIa havingthe level of the internal operating power supply VDDI. The second I/Ocell 31 b operates as an input cell in response to an H level signalS02, and converts a signal INb, which is provided from the externalsemiconductor device via the pad 26 a, into a signal DIb having thelevel of the internal operating power supply VDDI.

The first and second logic circuits 32 and 33 form a control circuit 28,which selects one of the first I/O cell 31 a and the second I/O cell 31b, and activate the selected I/O cell.

The I/O circuit 25 a further includes a selector circuit 34 responsiveto a voltage selection signal VSEL0. The selector circuit 34 is providedwith a signal DIa from the first I/O cell 31 a, and with a signal DIbfrom the second I/O cell 31 b. The selector circuit 34 selects one ofthe signal DIa and the signal DIb in response to a voltage selectionsignal VSEL0, and outputs an input signal DI0 having substantially thesame level as the level of the selected signal.

The following describes the first I/O cell 31 a.

The first I/O cell 31 a includes two logic circuits 35 a and 36 a, twolevel conversion circuits 37 a and 38 a, two input buffers 39 a and 40a, and two output transistors T1 a and T2 a.

Each of the logic circuits 35 a and 36 a is provided with a signal S01and an output signal DO0. The first logic circuit 35 a generates aninversion signal of the output signal DO0 in response to an L levelsignal S01, and generates an H level signal in response to an H levelsignal S01. The second logic circuit 36 a generates an inversion signalof the output signal DO0 in response to an L level signal S01, andgenerates an L level signal in response to an H level signal S01.

The first level conversion circuit 37 a converts an input signal havingthe level of the internal operating power supply VDDI (an output signalof the first logic circuit 35 a), into an input signal having the levelof the second high-potential power supply VDD2. The second levelconversion circuit 38 a converts an input signal having the level of theinternal operating power supply VDDI (an output signal of the secondlogic circuit 36 a), into an input signal having the level of the secondhigh-potential power supply VDD2.

The first output transistor T1 a is a PMOS (P-channel metal oxidesemiconductor) transistor. The second output transistor T2 a is an NMOS(N-channel metal oxide semiconductor) transistor. The first outputtransistor T1 a has its source connected to the first high-potentialpower supply VDD1, its drain connected to the second output transistorT2 a, its gate provided with an output signal of the first levelconversion circuit 37 a, and its back gate connected to the secondhigh-potential power supply VDD2. The second output transistor T2 a hasits source connected to the low-potential power supply VSS, its drainconnected to the first output transistor T1 a, its gate provided with anoutput signal of the second level conversion circuit 38 a, and its backgate connected to the low-potential power supply VSS.

The back gate of the first output transistor T1 a is set so that nocurrent flows through the first output transistor T1 a in a highimpedance state. To be specific, with the outputs of the first andsecond I/O cells 31 a and 31 b being connected to the pad 26 a, a signaloutput from the second I/O cell 31 b is provided to the drains of theoutput transistors T1 a and T2 a included in the first I/O cell 31 a.When the first output transistor T1 a, which is a PMOS transistor, hasits back gate connected to the first high-potential power supply VDD1,the potential of its back gate is lower than the potential of its drain.Thus, a forward diode is formed in a direction from the drain to theback gate of the first output transistor T1 a and current flows throughthe first output transistor T1 a. The back gate of the first outputtransistor T1 a is set so that the first output transistor T1 a is alsooff when a signal having the level of the second high-potential powersupply VDD2 passes through the pad 26 a. For example, a signal havingthe highest potential level among the signals passing through the pad 26a (the level of the second high-potential power supply VDD2 in thepresent embodiment) is provided to the back gate of the first outputtransistor T1 a. This prevents a current from flowing through the firstoutput transistor T1 a.

A node N1 between the first and second output transistors T1 a and T2 ais connected to the pad 26 a. The pad 26 a is connected to an inputterminal of the input buffer 39 a, which operates at the firsthigh-potential power supply VDD1. An output terminal of the input buffer39 a is connected to an input terminal of the input buffer 40 a, whichoperates at the internal operating power supply VDDI. A signal DIa isoutput from the input buffer 40 a.

In the first I/O cell 31 a, one of the first and second outputtransistors T1 a and T2 a is turned on in correspondence with the levelof an output signal DO0 when the signal S01 is at an L level. In thiscase, a signal OUTa having the level of the first high-potential powersupply VDD1 or the level of the low-potential power supply VSS isgenerated. In this way, the first I/O cell 31 a converts an outputsignal having the level of the internal operating power supply VDDI intoa signal OUTa having the level of the first high-potential power supplyVDD1.

The first I/O cell 31 a controls the node N1 between the first andsecond output transistors T1 a and T2 a to be in a high impedance statewhen the signal S01 is at an H level. The first I/O cell 31 a converts asignal having the level of the first high-potential power supply VDD1,which is provided to the pad 26 a from the external semiconductordevice, into a signal DIa having the level of the internal operatingpower supply VDDI.

The output levels of the first and second level conversion circuits 37 aand 38 a are set according to the level of a signal output from thesecond I/O cell 31 b. With the outputs of the first and second I/O cells31 a and 31 b being connected to the pad 26 a, a signal output from thesecond I/O cell 31 b is provided to the drains of the output transistorsT1 a and T2 a included in the first I/O cell 31 a. When a signal havingthe level of the first high-potential power supply VDD1 is provided tothe gate of the output transistor T1 a from the level conversion circuit37 a, the source-drain-gate potential causes the output transistor T1 ato be on, so that a current flows from the pad 26 a toward the firsthigh-potential power supply VDD1. To prevent such current from flowing,the potential of the gate of the output transistor T1 a is set at thelevel of the signal provided to the pad 26 a (at the level of the secondhigh-potential power supply VDD2 in the present embodiment). To bespecific, a signal having the level of the second high-potential powersupply VDD2 is provided to the gate of the output transistor T1 a fromthe level conversion circuit 37 a.

The following describes the second I/O cell 31 b.

The second I/O cell 31 b includes two logic circuits 35 b and 36 b, twolevel conversion circuits 37 b and 38 b, two input buffers 39 b and 40b, and two output transistors T1 b and T2 b.

Each of the logic circuits 35 b and 36 b is provided with a signal S02and an output signal DO0. The first logic circuit 35 b generates aninversion signal of the output signal DO0 in response to an L levelsignal S02, and generates an H level signal in response to an H levelsignal S02. The second logic circuit 36 b generates an inversion signalof the output signal DO0 in response to an L level signal S02, andgenerates an L level signal in response to an H level signal S02.

The first level conversion circuit 37 b converts an input signal havingthe level of the internal operating power supply VDDI (an output signalof the first logic circuit 35 b), into an input signal having the levelof the second high-potential power supply VDD2. The second levelconversion circuit 38 b converts an input signal having the level of theinternal operating power supply VDDI (an output signal of the secondlogic circuit 36 b), into an input signal having the level of the secondhigh-potential power supply VDD2.

The first output transistor T1 b is a PMOS transistor. The second outputtransistor T2 b is an NMOS transistor. The first output transistor T1 bhas its source connected to the second high-potential power supply VDD2,its drain connected to the second output transistor T2 b, its gateprovided with an output signal of the first level conversion circuit 37b, and its back gate connected to the second high-potential power supplyVDD2. The second output transistor T2 b has its source connected to thelow-potential power supply VSS, its drain connected to the first outputtransistor T1 b, its gate provided with an output signal of the secondlevel conversion circuit 38 b, and its back gate connected to thelow-potential power supply VSS. A node N2 between the first and secondoutput transistors T1 b and T2 b is connected to the pad 26 a.

The pad 26 a is connected to an input terminal of the input buffer 39 b,which operates at the second high-potential power supply VDD2. An outputterminal of the input buffer 39 b is connected to an input terminal ofthe input buffer 40 b, which operates at the internal operating powersupply VDDI. A signal DIb is output from the input buffer 40 b.

In the second I/O cell 31 b, one of the first and second outputtransistors T1 b and T2 b is turned on in correspondence with the levelof an output signal DO0 when the signal S02 is at an L level. In thiscase, a signal OUTb having the level of the second high-potential powersupply VDD2 or the level of the low-potential power supply VSS isgenerated. In this way, the second I/O cell 31 b converts an outputsignal having the level of the internal operating power supply VDDI intoa signal having the level of the second high-potential power supplyVDD2.

The second I/O cell 31 b controls the node N2 between the first andsecond output transistors T1 b and T2 b to be in a high impedance statewhen the signal S02 is at an H level. Here, the second I/O cell 31 bconverts a signal having the level of the second high-potential powersupply VDD2, which is provided to the pad 26 a from the externalsemiconductor device, into a signal DIb having the level of the internaloperating power supply VDDI.

FIG. 5 is a schematic diagram showing part of the semiconductor device21.

The power supply wirings 24 a to 24 c described with reference to FIG. 2are formed on the periphery of the semiconductor device 21. Also, thepad 26 a shown in FIG. 4, and the I/O circuit 25 a corresponding to thepad 26 a are arranged on the periphery of the semiconductor device 21.The first and second I/O cells 31 a and 31 b included in the I/O circuit25 a are arranged along the power supply wirings 24 a to 24 c.

The output transistors T1 a and T2 a included in the first I/O cell 31 aare arranged in the direction perpendicular to the power supply wirings24 a to 24 c. The output transistors T1 b and T2 b included in thesecond I/O cell 31 b are arranged in the direction perpendicular to thepower supply wirings 24 a to 24 c.

The output transistor T1 a, which is a PMOS transistor (referred to as“Pch” in FIG. 5), is connected to the wiring 24 a for providing thefirst high-potential power supply VDD1, via a contactor 41 a. The outputtransistor T1 b, which is a PMOS transistor, is connected to the wiring24 b for providing the second high-potential power supply VDD2, via acontactor 41 b. The output transistors T2 a and T2 b, which are NMOStransistors (referred to as “Nch”), are connected to the wiring 24 c forproviding the low-potential power supply VSS, via contactors 41 c and 41d, respectively.

In the semiconductor device 21, the power supply wiring 24 a isconnected to a pad 44 a via a contactor 42 a and wiring 43 a, the powersupply wiring 24 b is connected to a pad 44 b via a contactor 42 b andwiring 43 b, and the power supply wiring 24 c is connected to a pad 44 cvia a contactor 42 c and wiring 43 c. A device external to thesemiconductor device 21 provides the pads 44 a, 44 b, and 44 c, viawires, with the power supplies VDD1, VDD2, and VSS respectively.

At least one of the high-potential power supplies VDD1 and VDD2 may notbe provided from the external device. For example, the semiconductordevice 21 may be provided with the first high-potential power supplyVDD1, and a voltage step-up circuit included in the internal circuit 23may generate the second high-potential power supply VDD2 based on thefirst high-potential power supply VDD1. Alternatively, the internalcircuit 23 may generate the first and second high-potential powersupplies VDD1 and VDD2 based on the internal operating power supplyVDDI, which is the operating voltage of the internal circuit 23.

The following describes the operation of the I/O circuit 25 a.

When the control signal CIO is at an L level and the voltage selectionsignal VSEL is at an L level, the first and second output transistors T1a and T2 a in the first I/O cell 31 a are controlled on and offaccording to the level of the output signal DO0. The second I/O cell 31b sets the node N2 between the first and second output transistors T1 band T2 b in a high impedance state (turns off both the transistors T1 band T2 b). The I/O circuit 25 a operates using, as the operatingvoltage, the first high-potential power supply VDD1, which is connectedto the source of the output transistor T1 a included in the first I/Ocell 31 a.

When the control signal CIO is at an L level and the voltage selectionsignal VSEL is at an H level, the first I/O cell 31 a sets the node N1between the first and second output transistors T1 a and T2 a in a highimpedance state (turns off both the transistors T1 a and T2 a). Thefirst and second output transistors T1 b and T2 b in the second I/O cell31 b are controlled on and off according to the level of the outputsignal DO0. The I/O circuit 25 a operates using, as the operatingvoltage, the second high-potential power supply VDD2, which is connectedto the source of the output transistor T1 b included in the second I/Ocell 31 b.

When the control signal CIO is at an H level, the first I/O cell 31 asets the node N1 between the first and second output transistors T1 aand T2 a in a high impedance state, and the second I/O cell 31 b setsthe node N2 between the first and second output transistors T1 b and T2b in a high impedance state. The selector circuit 34 generates a signalDI0 having substantially the same level as the level of the signals DIaand DIb, which are output from one of the first and second I/O cells 31a and 31 b, in response to the voltage selection signal VSEL.

The semiconductor device 21 of the first embodiment has the advantagesdescribed below.

The I/O circuit 25 a includes the first I/O cell 31 a that operates atthe first high-potential power supply VDD1 and the second I/O cell 31 bthat operates at the second high-potential power supply VDD2. Accordingto the voltage selection signal VSEL0, the I/O circuit 25 a selectivelyactivates the first and second I/O cells 31 a and 31 b. A signalaccording to the operating voltage of the selected one of the first andsecond I/O cells 31 a and 31 b (the first high-potential power supplyVDD1 or the second high-potential power supply VDD2) is generated. As aresult, the operating voltage of the I/O circuit 25 a is easily changed.

The following describes a semiconductor device according to a secondembodiment of the present invention, with reference to FIGS. 6 and 7.

FIG. 6 is a circuit diagram of an I/O circuit 51 included in thesemiconductor device according to the second embodiment. The I/O circuit51 is replaceable with each of the I/O circuits 25 a to 25 e shown inFIG. 3.

The I/O circuit 51 includes five logic circuits 52 to 56, three levelconversion circuits 57 to 59, two input buffers 60 and 61, and threeoutput transistors T3 a to T3 c. The logic circuits 52 to 56 form acontrol circuit 49.

Each of the first and second logic circuits 52 and 53 is provided with avoltage selection signal VSEL0 and a control signal CIO. The first logiccircuit 52 generates a signal S11 having substantially the same level asthe level of a control signal CIO in response to an L level voltageselection signal VSEL0, and generates an H level signal S11 in responseto an H level voltage selection signal VSEL0. The second logic circuit53 generates a signal S12 having substantially the same level as thelevel of a control signal CIO in response to an H level voltageselection signal VSEL0, and generates an H level signal S12 in responseto an L level voltage selection signal VSEL0.

The third logic circuit 54 is provided with a signal S11 and an outputsignal DO0. The third logic circuit 54 generates an inversion signal ofthe output signal DO0 in response to an L level signal S11, andgenerates an H level signal in response to an H level signal S11. Thefirst level conversion circuit 57 converts an input signal having thelevel of the internal operating power supply VDDI (an output signal ofthe third logic circuit 54), into an input signal having the level ofthe second high-potential power supply VDD2, and provides the firstoutput transistor T3 a with the input signal resulting from theconversion.

The fourth logic circuit 55 is provided with a signal S12 and an outputsignal DO0. The fourth logic circuit 55 generates an inversion signal ofthe output signal DO0 in response to an L level signal S12, andgenerates an H level signal in response to an H level signal S12. Thesecond level conversion circuit 58 converts an input signal having thelevel of the internal operating power supply VDDI (an output signal ofthe fourth logic circuit 55), into an input signal having the level ofthe second high-potential power supply VDD2, and provides the secondoutput transistor T3 b with the input signal resulting from theconversion.

The fifth logic circuit 56 is provided with a control signal CIO and anoutput signal DO0. The fifth logic circuit 56 generates an inversionsignal of the output signal DO0 in response to an L level control signalCIO, and generates an L level signal in response to an H level controlsignal CIO. The third level conversion circuit 59 converts an inputsignal having the level of the internal operating power supply VDDI (anoutput signal of the fifth logic circuit 56), into an input signalhaving the level of the second high-potential power supply VDD2, andprovides the third output transistor T3 c with the input signalresulting from the conversion.

The first output transistor T3 a is a PMOS transistor. The first outputtransistor T3 a has its source connected to the first high-potentialpower supply VDD1, its gate provided with an output signal of the firstlevel conversion circuit 57, and its back gate connected to the secondhigh-potential power supply VDD2. In this way, the first outputtransistor T3 a is a transistor connected to a high-potential powersupply.

The second output transistor T3 b is a PMOS transistor. The secondoutput transistor T3 b has its source connected to the secondhigh-potential power supply VDD2, its gate provided with an outputsignal of the second level conversion circuit 58, and its back gateconnected to the second high-potential power supply VDD2. In this way,the second output transistor T3 b is a transistor connected to ahigh-potential power supply.

The third output transistor T3 c is an NMOS transistor. The third outputtransistor T3 c has its source connected to the low-potential powersupply VSS, its gate provided with an output signal of the third levelconversion circuit 59, and its back gate connected to the low-potentialpower supply VSS. In this way, the third output transistor T3 c is atransistor connected to a low-potential power supply.

The drains of the first to third output transistors T3 a to T3 c areconnected to one another. A node N11 between the drains of the first tothird output transistors T3 a to T3 c is connected to a pad 26 a. Thepad 26 a is connected to an input terminal of the input buffer 60, whichoperates at the second high-potential power supply VDD2. An outputterminal of the input buffer 60 is connected to an input terminal of theinput buffer 61, which operates at the internal operating power supplyVDDI. The input buffer 61 generates a signal DI0.

The following describes the operation of the I/O circuit 51.

When the control signal CIO is at an L level and the voltage selectionsignal VSEL is at an L level, the I/O circuit 51 turns off the secondoutput transistor T3 b, and turns on or off the first and third outputtransistors T3 a and T3 c according to the level of the output signalDO0. Thus, the I/O circuit 51 operates as an output circuit using, asthe operating voltage, the first high-potential power supply VDD1, whichis connected to the source of the first output transistor T3 a. The I/Ocircuit 51 generates a signal having the level of the firsthigh-potential power supply VDD1 or the level of the low-potential powersupply VSS.

When the control signal CIO is at an L level and the voltage selectionsignal VSEL is at an H level, the I/O circuit 51 turns off the firstoutput transistor T3 a, and turns on or off the second and third outputtransistors T3 b and T3 c according to the level of the output signalDO0. Thus, the I/O circuit 51 operates as an output circuit using, asthe operating voltage, the second high-potential power supply VDD2,which is connected to the source of the second output transistor T3 b.The I/O circuit 51 generates a signal having the level of the secondhigh-potential power supply VDD2 or the level of the low-potential powersupply VSS.

When the control signal CIO is at an H level, the I/O circuit 51 turnsoff the output transistors T3 a to T3 c, and sets the node N11 in a highimpedance state. The first input buffer 60 converts a signal provided tothe pad 26 a, into a signal having the level of the secondhigh-potential power supply VDD2 or into a signal having the level ofthe low-potential power supply VSS. The second input buffer 61 convertsan input signal into a signal having the level of the internal operatingpower supply VDDI or into a signal having the level of the low-potentialpower supply VSS, to generate a signal DI0.

FIG. 7 is a schematic diagram showing part of a semiconductor device 62.

A plurality of (three in the figure) I/O circuits 51 and pads 26 a arearranged on the periphery of the semiconductor device 62. Power supplywirings 24 a to 24 c are arranged on the I/O circuits 51. The outputtransistors T3 a to T3 c included in each I/O circuit 51 are arranged inthe direction perpendicular to the power supply wirings 24 a to 24 c.

The wiring 24 a for providing the first high-potential power supply VDD1is arranged above the output transistors T3 a, which are PMOStransistors (referred to as “Pch” in FIG. 7). Each output transistor T3a is connected to the wiring 24 a via a contactor 63 a. The wiring 24 bfor providing the second high-potential power supply VDD2 is arrangedabove the output transistors T3 b, which are PMOS transistors. Eachoutput transistor T3 b is connected to the wiring 24 b via a contactor63 b. The wiring 24 c for providing the low-potential power supply VSSis arranged above the output transistors T3 c, which are NMOStransistors (referred to as “Nch” in FIG. 7). Each output transistor T3c is connected to the wiring 24 c via a contactor 63 c.

Although the output transistors T3 a to T3 c included in each I/Ocircuit 51 are shown as adjacent to one another in FIG. 7, the outputtransistors T3 a to T3 c are actually formed in different wells, whichare isolated from one another by their conductivity types and powersupply voltages. The output transistors T3 a arranged along the powersupply wiring 24 a have the same conductivity type, and operate at thesame power supply voltage. For example, each of the output transistorsT3 a arranged along the power supply wiring 24 a is a PMOS transistor,and has its source connected to the first high-potential power supplyVDD1 (refer to FIG. 6). Thus, the output transistors T3 a are formed inone well.

In the same manner, the output transistors T3 b arranged along the powersupply wiring 24 b have the same conductivity type, and operate at thesame power supply voltage. Thus, the output transistors T3 b are formedin one well. Further, the output transistors T3 c arranged along thepower supply wiring 24 c have the same conductivity type, and operate atthe same power supply voltage. Thus, the output transistors T3 c areformed in one well.

The output transistors T3 a to T3 c included in the I/O circuits 51 arearranged along the power supply wirings 24 a to 24 c in correspondencewith their conductivity types and power supply voltages. The outputtransistors T3 a arranged in a row along the power supply wiring 24 aare formed in one well. The output transistors T3 b arranged in a rowalong the power supply wiring 24 b are formed in one well. The outputtransistors T3 c arranged in a row along the power supply wiring 24 care formed in one well. These three wells in which the outputtransistors T3 a to T3 c are respectively formed are actually isolatedfrom one another.

In this way, the output transistors T3 a to T3 c are formed so that theoutput transistors in the same row are in the same well. This increasesthe area of wells, compared with conventional examples. Thus, thesemiconductor device 62 has an improved electrostatic discharge (ESD)withstand voltage. The ESD withstand voltage depends on the area ofwells. To be specific, the ESD withstand voltage decreases as the areaof the wells decreases.

In the second embodiment, the PMOS transistors are designed to be largerthan the NMOS transistors. Compared with the I/O circuit 25 a of thefirst embodiment, the I/O circuit 51 of the second embodiment has anarrower width in the direction where the power supply wirings 24 a to24 c extend, and has a wider width in the direction perpendicular to thepower supply wirings 24 a to 24 c. Thus, the I/O circuit 51 has anarrower width with respect to one pad 26 a compared with the outputcircuit of the first embodiment. This structure enables a larger numberof I/O circuits 51 to be arranged on the periphery of the semiconductordevice 62, compared with the semiconductor device of the firstembodiment.

The semiconductor device of the second embodiment has the advantagesdescribed below.

(1) The I/O circuit 51 includes the first and second output transistorsT3 a and T3 b, which are PMOS transistors, and the third outputtransistor T3 c, which is an NMOS transistor. The drain of the thirdoutput transistor T3 c is connected to the drains of the first andsecond output transistors T3 a and T3 b. The third output transistor T3c is commonly used by the first and second output transistors T3 a andT3 b. This structure prevents an increase in the circuit area of thesemiconductor device.

(2) The first to third output transistors T3 a to T3 c included in theI/O circuit 51 are arranged in the direction perpendicular to the powersupply wirings 24 a to 24 c. The I/O circuits 51 are arranged along thepower supply wirings 24 a to 24 c. The output transistors T3 a to T3 care arranged so that the transistors with the same conductivity type arein the same row along the power supply wirings. Among the outputtransistors T3 a to T3 c included in the I/O circuits 51, thetransistors with the same conductivity type are formed in the same well.This structure increases the area of each well, and improves the ESDwithstand voltage of the semiconductor device.

The following describes a semiconductor device according to a thirdembodiment of the present invention, with reference to FIGS. 8 and 9.

FIG. 8 is a circuit diagram of an I/O circuit 71. The I/O circuit 71 isreplaceable with, for example, each of the I/O circuits 25 a to 25 eshown in FIG. 3.

The I/O circuit 71 includes five logic circuits 52 to 56, three levelconversion circuits 57 to 59, two input buffers 60 and 61, three outputtransistors T3 a to T3 c, a delay circuit 72, a logic circuit 73, andtwo selector circuits 74 and 75.

The delay circuit 72 receives an output signal DO0, and generates adelay signal S21 by delaying the output signal DO0 by a predeterminedtime Δt.

The logic circuit 73 is provided with an output signal DO0 and a delaysignal S21. The logic circuit 73 generates a signal S22 havingsubstantially the same level as the level of the output signal DO0 whenthe delay signal S21 is at an L level, and generates an L level signalS22 when the delay signal S21 is at an H level. The delay signal S21 isa signal resulting from delaying the output signal DO0 by thepredetermined time Δt. As shown in FIG. 9, the logic circuit 73generates a signal S22, which is maintained at an H level only for thepredetermined time Δt from the rise of the output signal DO0.

The first selector circuit 74 is provided with a mode signal MODE as acontrol signal. The first selector circuit 74 is also provided with asignal S22 and an output signal of the third logic circuit 54. The firstselector circuit 74 selects the signal S22 or the output signal of thethird logic circuit 54 according to the mode signal MODE, and generatesa signal having substantially the same level as the level of theselected signal. For example, the first selector circuit 74 selects thesignal S22 in response to an H level mode signal MODE, and selects theoutput signal of the third logic circuit 54 in response to an L levelmode signal MODE.

The second selector circuit 75 is provided with a mode signal MODE as acontrol signal. The second selector circuit 75 is also provided with adelay signal S21 and an output signal of the fourth logic circuit 55.The second selector circuit 75 selects the delay signal 21 or the outputsignal of the fourth logic circuit 55 according to the mode signal MODE,and generates a signal having substantially the same level as the levelof the selected signal. For example, the second selector circuit 75selects the signal S21 in response to an H level mode signal MODE, andselects the output signal of the fourth logic circuit 55 in response toan L level mode signal MODE.

The mode signal MODE is used to reduce overshoot, which may occur at therise of an output signal. The mode signal MODE is set at an H level orat an L level by, for example, pulling up or pulling down a pad, whichis not shown.

When the mode signal MODE is set at an L level, the first selectorcircuit 74 selects the output signal of the third logic circuit 54, andthe second selector circuit 75 selects the output signal of the fourthlogic circuit 55. The first and second selector circuits 74 and 75respectively provide the output transistors T3 a and T3 b with theselected output signals. The delay circuit 72, the logic circuit 73, andthe selector circuits 74 and 75 included in the I/O circuit 71 form acontrol circuit 69, which switches the operating power supply to thefirst high-potential power supply VDD1 or to the second high-potentialpower supply VDD2 according to the voltage selection signal VSEL0, andcontrols the I/O circuit 71 to operate as an input circuit or as anoutput circuit according to the control signal CIO.

When the mode signal MODE is set at an H level, the first selectorcircuit 74 selects the signal S22 output from the logic circuit 73, andthe second selector circuit 75 selects the delay signal S21 output fromthe delay circuit 72. The first and second selector circuits 74 and 75respectively provide the output transistors T3 a and T3 b with theselected signals. As shown in FIG. 9, the signal S22 is maintained at anH level only for the predetermined time Δt in response to the rise ofthe output signal DO0 (time t1). The delay signal S21 is a signalresulting from delaying the output signal DO0 by the predetermined timeΔt.

The output transistors T3 a and T3 b are PMOS transistors. From time t1to time t2, the first output transistor T3 a is on in response to thesignal S22 whose level has been converted by the first level conversioncircuit 57, and the second output transistor T3 b is off in response tothe delay signal S21 whose level has been converted by the second levelconversion circuit 58. From time t2 to time t3, the first outputtransistor T3 a is off in response to the signal S22 whose level hasbeen converted, and the second output transistor T3 b is on in responseto the delay signal S21 whose level has been converted.

To be specific, when provided with an H level output signal DO0, the I/Ocircuit 71 first turns on the output transistor T3 a. When thepredetermined time Δt elapses, the I/O circuit 71 turns off the firstoutput transistor T3 a and turns on the second output transistor T3 b.The delay circuit 72 and the logic circuit 73 form a control circuitthat controls on and off of the output transistors T3 a and T3 b atdifferent timings.

The output transistors T3 a and T3 b being turned on and off cause thevoltage of the output signal OUT (the level at the pad 26 a) to firstrise from the level of the low-potential power supply VSS to the levelof the first high-potential power supply VDD1, and then rise from thelevel of the first high-potential power supply VDD1 to the level of thesecond high-potential power supply VDD2.

In this way, the voltage of the output signal OUT rises from the levelof the low-potential power supply VSS to the level of the secondhigh-potential power supply VDD2 in a stepwise manner. The potentialdifference between adjacent steps is smaller than the potentialdifference between the low-potential power supply VSS and the secondhigh-potential power supply VDD2. A first overshoot occurs when thevoltage of the output signal rises from the level of the low-potentialpower supply VSS to the level of the first high-potential power supplyVDD1. The level of the first overshoot corresponds to the voltagedifference between the first high-potential power supply VDD1, which isa targeted voltage level, and the highest level of the output signal OUTto which the overshoot has occurred. A second overshoot occurs when thevoltage of the output signal rises from the level of the firsthigh-potential power supply VDD1 to the level of the secondhigh-potential power supply VDD2. The level of the first overshoot andthe level of the second overshoot are both lower than the level of anovershoot occurring when the voltage of an output signal OL (refer toFIG. 9) rises from the level of the low-potential power supply VSS tothe level of the second high-potential power supply VDD2.

In addition to the advantages described in the second embodiment, thesemiconductor device of the third embodiment has the advantagesdescribed below.

The I/O circuit 71 turns on the first output transistor T3 a and thesecond output transistor T3 b at timings that differ by thepredetermined time Δt, according to the mode signal MODE. The firstoutput transistor T3 a is provided with the first high-potential powersupply VDD1. When the predetermined time Δt elapses, the second outputtransistor T3 b is provided with the second high-potential power supplyVDD2. The voltage of the output signal OUT of the I/O circuit 71 risesfrom the level of the low-potential power supply VSS to the level of thesecond high-potential power supply VDD2 in a stepwise manner. Thus, anovershoot occurring with the output signal OUT is smaller than anovershoot occurring with the output signal OL whose voltage directlyrises from the level of the low-potential power supply VSS to the levelof the second high-potential power supply VDD2.

In the above embodiments, the operating voltage of the I/O circuits 25 ato 25 e, 51, and 71 is set using the voltage selection signal VSEL, andthe level of an external output signal is changed to the firsthigh-potential power supply VDD1 or to the second high-potential powersupply VDD2. However, the signal level may be changed by other methods.

For example, a semiconductor device 81 shown in FIG. 10 includes I/Ocircuits 82, each of which includes a first and second I/O cells 31 aand 31 b having the same structure as the corresponding components inthe first embodiment. The I/O circuit 82 does not include a componentcorresponding to the control circuit 28 in the I/O circuit 25 a of thefirst embodiment. The first and second I/O cells 31 a and 31 b receivean output signal D00, and operate at the same timing.

The first I/O cell 31 a is connected to a pad 83 a. The second I/O cell31 b is connected to a pad 83 b. To be specific, the node N1 between theoutput transistors T1 a and T2 a included in the first I/O cell 31 ashown in FIG. 4 is connected to the pad 83 a shown in FIG. 10, and thenode N2 between the output transistors T1 b and T2 b included in thesecond I/O cell 31 b is connected to the pad 83 b shown in FIG. 10.Thus, the first pad 83 a is provided with a signal having the level ofthe first high-potential power supply VDD1 from the first I/O cell 31 a,and the second pad 83 b is provided with a signal having the level ofthe second high-potential power supply VDD2 from the second I/O cell 31b. One of the two pads 83 a and 83 b is connected to anothersemiconductor device. In FIG. 10, for example, a lead frame 84 isselectively connected to the first and second pads 83 a and 83 b via awire 85. This structure enables the operating voltage of the I/O circuitto be changed without requiring a mask for forming the semiconductordevice 81 to be newly created, and prevents an increase in themanufacturing cost of the semiconductor device. Also, this structureonly requires the connection target of the wire 85 to be changed in theprocess for connecting the wire 85. This structure does not increase thenumber of manufacturing processes, and prevents an increase in thenumber of days required to manufacture the semiconductor device.

In the first embodiment, the first and second I/O cells 31 a and 31 bmay be arranged in the direction perpendicular to the power supplywirings 24 a to 24 c as shown in FIG. 11, instead of being arrangedalong the power supply wirings 24 a to 24 c. In this case, a powersupply wiring 86 a for the low-potential power supply VSS needs to bearranged in the output transistor (NMOS transistor) T2 a included in theI/O cell 31 a, and a power supply wiring 86 b for the low-potentialpower supply VSS needs to be arranged in the output transistor (NMOStransistor) T2 b included in the I/O cell 31 b. However, this structureenables the output transistor (PMOS transistors) T1 a included in thefirst I/O cell 31 a to be arranged along the power supply wiring 24 afor the first high-potential power supply VDD1, and the outputtransistor (PMOS transistor) T1 b included in the second I/O cell 31 bto be arranged along the power supply wiring 24 b for the secondhigh-potential power supply VDD2. As in the second embodiment, thisstructure enables the output transistors T1 a included in different I/Ocells to be formed in one well, and the output transistors T1 b includedin different I/O cells to be formed in another well. This structureenables the semiconductor device including the I/O cells 31 a and 31 baccording to the first embodiment, too, to have an improved ESDwithstand voltage.

In the above embodiments, the voltage selection signal VSEL may begenerated inside the semiconductor device.

For example, a semiconductor device 91 shown in FIG. 12(a), whichincludes the I/O circuit 51, includes a signal generation circuit 92.The signal generation circuit 92 includes a fuse F1. A first terminal ofthe fuse F1 is connected to the internal operating power supply VDDI,and a second terminal of the fuse F1 is connected to a first terminal ofa resistor R1. A second terminal of the resistor R1 is connected to afirst terminal of a resistor R2. A second terminal of the resistor R2 isconnected to the low-potential power supply VSS. A node N3 between theresistors R1 and R2 is connected to an inverter circuit 93. A voltageselection signal VSEL is output from the inverter circuit 93. The fuseF1 is broken when an output signal S31 of the input buffer 60 is at thelevel of the second high-potential power supply VDD2, and is not brokenwhen the output signal S31 is at the level of the first high-potentialpower supply VDD1.

The fuse F1 is not broken when a signal having the level of the firsthigh-potential power supply VDD1 is provided from a semiconductor device(not shown) connected to the pad 26 a. In this case, the signalgeneration circuit 92 generates the voltage selection signal VSEL at anL level. The I/O circuit 51 operates using the first high-potentialpower supply VDD1 as its driving power supply. The fuse F1 is brokenwhen a signal having the level of the second high-potential power supplyVDD2 is provided to the pad 26 a. In this case, the signal generationcircuit 92 generates an H level voltage selection signal VSEL. The I/Ocircuit 51 operates using the second high-potential power supply VDD2 asits driving power supply.

In this way, the operating voltage of the I/O circuit 51 isautomatically set according to a signal provided from an externalsemiconductor device connected to the pad 26 a. Also, the voltageselection signal VSEL is generated to have a level according to whetherthe fuse F1 is broken or not. This eliminates the need for the initialsetting.

The signal generation circuit may, for example, have a structure asshown in FIG. 12(b). A signal generation circuit 92 a includes two fusesF1 and F2, and resistors R1 and R2. The fuse F1 is connected to theinternal operating power supply VDDI. The fuse F2 is connected to thelow-potential power supply VSS. The resistors R1 and R2 are connectedbetween the fuses F1 and F2. The fuse F1 is broken by the current from asignal S31 when the first high-potential power supply VDD1 is used asthe operating power supply. The fuse F2 is broken by the current from asignal S32 when the second high-potential power supply VDD2 is used asthe operating power supply. It is preferable that the current breakingthe first fuse F1 and the current breaking the second fuse F2 beprovided via different pads. Providing the currents in this way reliablyenables the first and second fuses F1 and F2 to be broken.Alternatively, the fuses F1 and F2 may be broken by a laser or the likebefore the semiconductor device is assembled.

The signal generation circuits 92 and 92 a may be integrated in the I/Ocircuits 25 a to 25 e in the first embodiment, or in the I/O circuit 71in the third embodiment.

In the above embodiments, information for setting the voltage selectionsignal VSEL (setting information) may be stored, and the operatingvoltage of the I/O circuit may be set based on the setting information.It is preferable that the setting information be stored in a registerformed by a flip-flop etc., or in such a memory as an SRAM (staticrandom access memory).

For example, a semiconductor device 101 shown in FIG. 13 includes aplurality of registers 103, each connected to one of a plurality of I/Ocircuits 102 (e.g., I/O circuits having the same structure as thestructure of the I/O circuit 51 in the second embodiment). The registers103 are connected to a bus 105. A CPU core (CORE) 106, which functionsas an internal circuit, and an I/O circuit 107 are connected to the bus105. The I/O circuit 107 is connected to a pad 108. The I/O circuit 107may have a different structure from or the same structure as thestructure of the I/O circuits 102. The CPU core 106 receives settinginformation for each I/O circuit 102 via the pad 108, the I/O circuit107, and the bus 105, and stores the setting information into thecorresponding register 103. Each register 103 provides a voltageselection signal VSEL based on the setting information stored therein,to the corresponding I/O circuit 102. In this way, appropriatelychanging setting information stored in each register 103 enables theoperating voltage of each I/O circuit 102 to be easily changed. Also,the setting information for each of the I/O circuits 102 is provided viaone pad 108. In this way, this structure prevents an increase in thenumber of pads.

The method of generating a voltage selection signal based on settinginformation may be applied to a semiconductor device that does notinclude the bus 105. Further, one register 103 may be connected to aplurality of I/O circuits 102 as shown in FIG. 14, or one register maybe connected to one I/O block. In this way, the operating voltage of aplurality of I/O circuits may be set based on setting information storedin one register. This structure prevents an increase in the number ofregisters, and prevents an increase in the circuit area of thesemiconductor device.

A shift register may be used as such a register for storing settinginformation. For example, a semiconductor device 111 shown in FIG. 15(a)includes a shift register 112. The shift register 112 includes aplurality of registers 113 connected in series. Each register 113 isprovided with a clock signal CLK via a pad 114 and an I/O circuit 115.The clock signal CLK is used as a shift-pulse signal. The shift register112 sequentially transfers setting information, which is provided viathe pad 116 and the I/O circuit 117, according to the clock signal CLK.In this way, the setting information is provided as serial data from onepad, and the setting information is written to the shift register. Thisstructure prevents an increase in the number of pads, and prevents anincrease in the circuit area of the semiconductor device 111. Also, thearea occupied by the registers 113 is smaller than the area occupied bythe bus 105 and the registers 103 shown in FIG. 13. This structureprevents an increase in the chip area.

As shown in FIG. 15(b), a semiconductor device 121 corresponding to aboundary-scan design may include a shift register 122. The shiftregister 122 includes a plurality of registers 123, which are connectedbetween pads 124 and 125. Data is externally written to the shiftregister 122, to set the operation of each I/O circuit 126. Also, datastored in the shift register 123 is externally read, to check the stateof each I/O circuit 126. To be specific, in the semiconductor device121, data is externally written to the shift register 122, to set theoperating voltage of each I/O circuit 126. Also, setting informationstored in the shift register 122 is externally read.

The pad for each I/O circuit may be used to provide setting information.For example, a semiconductor device 131 as shown in FIG. 16(a) includesa pad 26 a, which is connected not only to an I/O circuit 51 but also toa flip-flop (FF) 132 as a register. A clock input terminal of theflip-flop 132 is provided with a fetch signal SET. It is preferable thatthe fetch signal SET be an externally provided signal, a reset signalused at power-on (a signal provided from a reset terminal or a signalgenerated by an internal circuit), and an output signal of an internalcounter circuit. A pull-down resistor R11, or a pull-up resistor R12shown in FIG. 16(b) is connected to the pad 26 a for setting theoperating voltage of the I/O circuit 51.

When the I/O circuit 51 is set in a high impedance state, the pull-downresistor R11 or the pull-up resistor R12 connected to the pad 26 aprovides the flip-flop 132 with an L level or H level signal. Forexample, when the pull-down resistor R11 is connected, the flip-flop 132outputs an L level voltage selection signal VSEL, so that the I/Ocircuit 51 generates a signal having the level of the firsthigh-potential power supply VDD1. When the pull-up resistor R12 isconnected, the flip-flop 132 outputs an H level voltage selection signalVSEL, so that the I/O circuit 51 generates a signal having the level ofthe second high-potential power supply VDD2.

The semiconductor device 131 is not required to additionally include apad for providing the register (flip-flop 132) with setting information.This structure prevents an increase in the circuit area of thesemiconductor device 131. The semiconductor device 131 simply includesthe pull-down resistor R11 or the pull-up resistor R12 connected to thepad 26 a, and is realized by a simple structure. Further, the operatingvoltage of the I/O circuit 51 is changed simply by changing theconnection target of the resistor. In this way, the operating voltage iseasily changed.

In the above embodiments, the level of a signal provided to the pad froman external semiconductor device may be determined, and the operatingpower supply voltage may be set based on the determined signal level.

For example, a semiconductor device 141 shown in FIG. 17 includes ananalogue-to-digital (AD) converter 142. The AD converter 142 is ofone-bit output type, and has an input terminal connected to a pad 26 afor each I/O circuit 51. A switch circuit (analogue switch) 143 isconnected between the AD converter 142 and each pad 26 a. Each pad 26 ais connected to an external semiconductor device 144. The semiconductordevice 144 provides each pad 26 a with an H level signal. The ADconverter 142 performs A/D conversion of an input signal, byappropriately turning on and off the switch circuit 143 at the time ofdetermining the level of the signal. The AD converter 142 generates avoltage selection signal VSEL according to the conversion result. To bespecific, the AD converter 142 generates an L level voltage selectionsignal VSEL when provided with a signal having the level of the firsthigh-potential power supply VDD1 from the external semiconductor device144, and generates an H level voltage selection signal VSEL whenprovided with a signal having the level of the second high-potentialpower supply VDD2.

The switch circuit 143 may be controlled by an internal circuit (e.g., aCPU). The internal circuit, such as a CPU, may determine the level of asignal provided from the external semiconductor device 144 based on amultiple-bit digital signal output from the AD converter, and providethe corresponding I/O circuit 51 with a voltage selection signal VSELbased on the determination result. Also, one bit included in themultiple-bit digital signal output from the AD converter may be used torepresent the voltage selection signal VSEL.

A semiconductor device 151 shown in FIG. 18 includes two input buffers152 and 153 connected to a pad 26 a. The input buffers 152 and 153function as comparison means. A comparator may be used as the comparisonmeans.

Each of the input buffers 152 and 153 has a different threshold voltage.The threshold voltages of the input buffers 152 and 153 are setaccording to the level of a signal passing through the pad 26 a, thatis, the operating power supply voltage at which the I/O circuit 51operates (the first high-potential power supply VDD1 and the secondhigh-potential power supply VDD2), or the level of a signal providedfrom an external semiconductor device connected to the pad 26 a.

In detail, the level of a signal passing through the pad 26 a is eitherthe level of the first high-potential power supply VDD1 or the level ofthe second high-potential power supply VDD2. To determine such a signallevel, an input buffer having a threshold voltage set between the twolevels is required. The above determination basically requires one inputbuffer. With only one input buffer, however, whether the determinationresult is correct remains unknown. For example, when a signal having thelevel lower than the threshold voltage is provided, the level of thesignal may be erroneously determined as the level of the firsthigh-potential power supply VDD1. It is preferable that another inputbuffer having a threshold voltage lower than the level of the firsthigh-potential power supply VDD1 be used.

To be specific, a first threshold voltage Vth1 of the first input buffer152 is set at a voltage lower than the voltage of the firsthigh-potential power supply VDD1 (e.g., at 1.5 V), and a secondthreshold voltage Vth2 of the second input buffer 153 is set at avoltage between the voltages of the first high-potential power supplyVDD1 and the second high-potential power supply VDD2 (e.g., at 2 V).

The output terminal of the first input buffer 152 is connected to afirst flip-flop (FF) 154. The output terminal of the second input buffer153 is connected to a second flip-flop (FF) 155. Each of the first andsecond flip-flops 154 and 155 latches an input signal in response to afetch signal SET, and outputs the latched signal. An internal circuit,which is not shown (e.g., a CPU), determines the level of a signalprovided to the pad 26 a based on the latched output signals of thefirst and second flip-flops 154 and 155, and provides the I/O circuit 51with a voltage selection signal VSEL based on the determination result.

The output signals of the first and second flip-flops 154 and 155(output signals of the first and second input buffers 152 and 153) arein combinations shown in FIG. 19. In FIG. 19, “A” represents the outputsignal of the first flip-flop 154, “B” the output signal of the secondflip-flop 155, “1” an H level, and “0” an L level.

When the output signals (A, B) are both at an L level (0, 0), the inputlevels of the first and second input buffers 152 and 153 are lower thanthe first threshold voltage Vth1. In other words, the level at the pad26 a is determined to be lower than the level of the firsthigh-potential power supply VDD1. In this case, there is a wait time fordetermination.

When the output signals (A, B) are at an H level and at an L level (1,0) respectively, the input level of the first input buffer 152 is higherthan the first threshold voltage Vth1, and the input level of the secondinput buffer 153 is lower than the second threshold voltage Vth2. Inother words, the level at the pad 26 a is determined as the level of thefirst high-potential power supply VDD1. In this case, for example, an Llevel voltage selection signal VSEL is generated, to enable the I/Ocircuit 51 to operate at the first high-potential power supply VDD1.

When the output signals (A, B) are both at an H level (1, 1), the inputlevel of the first input buffer 152 is higher than the first and secondthreshold voltages Vth1 and Vth2. In other words, the level at the pad26 a is determined as the level of the second high-potential powersupply VDD2. In this case, for example, an H level voltage selectionsignal VSEL is generated, to enable the I/O circuit 51 to operate at thesecond high-potential power supply VDD2.

A semiconductor device 161 shown in FIG. 20 includes first and secondinput buffers 162 and 163. An input terminal of the first input buffer162 is connected to a pad 164. An output terminal of the first inputbuffer 162 is connected to an input terminal of the second input buffer163. The first input buffer 162 has a high-potential power supplyterminal connected to the second high-potential power supply VDD2, and alow-potential power supply terminal connected to the firsthigh-potential power supply VDD1. The second input buffer 163 has ahigh-potential power supply terminal connected to the secondhigh-potential power supply VDD2, and a low-potential power supplyterminal connected to the low-potential power supply VSS. The first andsecond input buffers 162 and 163 connected in this way function as levelconverters for shifting a low potential level of a signal provided tothe pad 164. To be specific, when the level of a signal S4 is the levelof the second high-potential power supply VDD2, the first input buffer162 outputs an H level (level of the second high-potential power supplyVDD2) signal, and the second input buffer 163 outputs an H level (levelof the second high-potential power supply VDD2) voltage selection signalVSEL. When the level of the signal S4 is the level of the firsthigh-potential power supply VDD1, the first input buffer 162 outputs anL level (level of the first high-potential power supply VDD1) signal,and the second input buffer 163 outputs an L level (level of thelow-potential power supply VSS) voltage selection signal VSEL. With suchinput buffers 162 and 163 being included in the semiconductor device,the operating voltage of the I/O circuit is easily set according to thelevel of a signal provided to the pad 164.

In the third embodiment (FIG. 8), an I/O circuit 71 a shown in FIG. 21may be used. The I/O circuit 71 a includes an AND circuit 171, which isconnected between a delay circuit 72 and a second selector circuit 75.The AND circuit 171 provides the second selector circuit 75 with asignal S21 a indicating an operational result of the logical AND of anoutput signal of the delay circuit 72 and an output signal DO0. Withthis structure, the signal S21 a falls at the same timing as the outputsignal DO0 as shown in FIG. 22. The operation time of the I/O circuit 71a is substantially the same as the operation time of an I/O circuit thatis not designed to reduce overshoot. This structure enables the I/Ocircuit 71 a to operate at the same timing as an I/O circuit that is notdesigned to reduce overshoot.

In the third embodiment (FIG. 8), an I/O circuit 71 b shown in FIG. 23may be used. The I/O circuit 71 b includes, in addition to the circuitelements shown in FIG. 8, a logic circuit (OR circuit) 172, a logiccircuit (NOR circuit) 173, and a selector circuit 174. The OR circuit172 receives an output signal DO0 and a signal S21. The NOR circuit 173receives an output signal S23 of the OR circuit 172 and a control signalCIO. The selector circuit 174 is connected between the fifth logiccircuit 56 and the level conversion circuit 59. The selector circuit 174is provided with a mode signal MODE as a control signal, an outputsignal of the fifth logic circuit 56, and an output signal of the NORcircuit 173. The selector circuit 174 selects the output signal of theNOR circuit 173 or the output signal of the fifth logic circuit 56 basedon the mode signal MODE, and generates a signal having substantially thesame level as the level of the selected signal. For example, the firstselector circuit 74 selects the output signal of the NOR circuit 173 inresponse to an H level mode signal MODE, and selects the output signalof the fifth logic circuit 56 in response to an L level mode signalMODE.

When the control signal CIO is at an L level, that is, when the I/Ocircuit 71 b generates an output signal OUT based on the output signalDO0, the fifth logic circuit 56 outputs an inversion signal of theoutput signal DO0, and the NOR circuit 173 outputs an inversion signalof the signal S23. The selector circuit 174 selects one of thesesignals, so that the output transistor T3 c is turned on or off based onthe selected signal.

FIG. 24 is a waveform diagram of the I/O circuit 71 b when the modesignal MODE is at an H level. From time t1 to time t2, the first outputtransistor T3 a is on in response to a signal S22. From time t2 to timet3, the second output transistor T3 b is on in response to a signal S21.From time t1 to time t3, the third output transistor T3 c is off inresponse to a logical AND operation signal S23, which indicates anoperational result of the logical AND of an output signal DO0 and asignal S21. In this way, one of the first to third output transistors T3a to T3 c is on.

In the I/O circuit 71 of the third embodiment shown in FIG. 8, the thirdoutput transistor T3 c is turned on or off in response to an outputsignal of the fifth logic circuit 56. The output signal of the fifthlogic circuit 56 has the inverted level of the output signal DO0. Thus,in the I/O circuit 71 of the third embodiment, the third outputtransistor T3 c is off from time t1 to time t4 at which the outputsignal DO0 shifts to an L level, and is on from time t4 to time t3.Thus, from time t4 to time t3, both the second output transistor T3 band the third output transistor T3 c are on, so that a flow-throughcurrent flows via the output transistors T3 b and T3 c.

Contrary to this, the third output transistor T3 c is off from time t1to time t3. In the I/O circuit 71 b shown in FIG. 23, no flow-throughcurrent flows. Thus, power consumption of the I/O circuit 71 b issmaller than that of the I/O circuit 71 of the third embodiment. In theI/O circuit 71 a shown in FIG. 21, the second output transistor T3 b isoff from the fall of the output signal DO0 to time t3 in response to thesignal S21 a. Like in the I/O circuit 71 b, no flow-through currentflows in the I/O circuit 71 a. Thus, power consumption of the I/Ocircuit 71 a is smaller than that of the I/O circuit 71 of the thirdembodiment.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Therefore, the presentexamples and embodiments are to be considered as illustrative and notrestrictive, and the invention is not to be limited to the details givenherein, but may be modified within the scope and equivalence of theappended claims.

1. A semiconductor device comprising: an internal circuit for generatingan output signal; and an output circuit, connected to the internalcircuit, for converting a voltage level of the output signal of theinternal circuit, the output circuit including: a plurality of outputcells for generating a plurality of level-converted output signalshaving different voltages, respectively; and a control circuit,connected to the plurality of output cells, for selecting one of theplurality of output cells according to a voltage selection signal. 2.The semiconductor device according to claim 1, wherein each of theplurality of output cells includes: a first output transistor connectedto a high-potential power supply; and a second output transistorconnected between a low-potential power supply and the first outputtransistor, the corresponding level-converted output signal beinggenerated at a node between the first output transistor and the secondoutput transistor, wherein the control circuit turns off the first andsecond output transistors of the output cells except for those of theoutput cell selected according to the voltage selection signal.
 3. Thesemiconductor device according to claim 1, wherein the plurality ofoutput cells include a first output cell and a second output cell, thefirst output cell includes: a first output transistor connected to afirst high-potential power supply; and a second output transistorconnected between a low-potential power supply and the first outputtransistor, a first level-converted output signal being generated at anode between the first output transistor and the second outputtransistor; and the second output cell includes: a third outputtransistor connected to a second high-potential power supply having avoltage higher than a voltage of the first high-potential power supply;and a fourth output transistor connected between the low-potential powersupply and the third output transistor, a second level-converted outputsignal being generated at a node between the third output transistor andthe fourth output transistor; and the control circuit selectivelyactivates the first and second output cells according to the voltageselection signal.
 4. The semiconductor device according to claim 3,wherein the semiconductor device includes a pad connecting the firstnode and the second node.
 5. The semiconductor device according to claim4, wherein the first output transistor has a back gate connected to thesecond high-potential power supply.
 6. The semiconductor deviceaccording to claim 1, further comprising: a signal generation circuit,connected to the output circuit, for generating the voltage selectionsignal, wherein the signal generation circuit includes a fuse that isbroken when the voltage selection signal for selecting a specific one ofthe plurality of output cells is generated.
 7. The semiconductor deviceaccording to claim 1, further comprising: a rewritable storage unit forstoring setting information of voltage; and a signal generation circuit,connected to the storage unit, for generating the voltage selectionsignal based on the stored setting information.
 8. The semiconductordevice according to claim 1, further comprising a plurality of inputbuffers, each having a different threshold voltage and generating abuffer output signal according to the threshold voltage, wherein anoperating voltage of a device connected to the semiconductor device isdetected based on the buffer output signal of each input buffer, and thevoltage selection signal is generated based on the detected operatingvoltage.
 9. The semiconductor device according to claim 1, furthercomprising: a first input buffer having a first low-potential powersupply terminal connected to a first high-potential power supply, and afirst high-potential power supply terminal connected to a secondhigh-potential power supply having a voltage higher than a voltage ofthe first high-potential power supply, wherein the first input bufferreceives an external signal and generates a buffer output signal; and asecond input buffer having a second low-potential power supply terminal,connected to a low-potential power supply, and a second high-potentialpower supply terminal, connected to the second high-potential powersupply, wherein the second input buffer receives the buffer outputsignal from the first input buffer and generates the voltage selectionsignal.
 10. A semiconductor device comprising: an internal circuit forgenerating an output signal; and an output circuit, connected to theinternal circuit, for converting a voltage level of the output signal ofthe internal circuit, the output circuit including: a plurality of firstoutput transistors connected to a plurality of high-potential powersupplies and having different voltage, respectively; a second outputtransistor connected to a low-potential power supply and to theplurality of first output transistors; and a control circuit forselecting one of the plurality of first output transistors according toa voltage selection signal, and causing the selected first outputtransistor and the second output transistor to perform a levelconversion operation.
 11. The semiconductor device according to claim10, further comprising wiring connected to the high-potential powersupplies, wherein the output circuit is one of a plurality of outputcircuits arranged along the wiring; and the plurality of first outputtransistors of each output circuit are arranged in a directionperpendicular to the wiring.
 12. The semiconductor device according toclaim 10, wherein the plurality of first output transistors include: afirst transistor connected to a first high-potential power supply; asecond transistor connected to a second high-potential power supplyhaving a voltage higher than a voltage of the first high-potential powersupply; and the second output transistor is connected between alow-potential power supply, the first transistor, and the secondtransistor.
 13. The semiconductor device according to claim 12, furthercomprising: a pad connected to a node between the second outputtransistor, the first transistor, and the second transistor.
 14. Thesemiconductor device according to claim 13, wherein the first transistorhas a back gate connected to the second high-potential power supply. 15.The semiconductor device according to claim 10, further comprising asignal generation circuit, connected to the output circuit, forgenerating the voltage selection signal, wherein the signal generationcircuit includes a fuse that is broken when the voltage selection signalfor selecting a specific one of the plurality of output cells isgenerated.
 16. The semiconductor device according to claim 10, furthercomprising: a rewritable storage unit for storing setting information ofvoltage; and a signal generation circuit, connected to the storage unit,for generating the voltage selection signal based on the stored settinginformation.
 17. The semiconductor device according to claim 10, furthercomprising a plurality of input buffers, each having a differentthreshold voltage and generating a buffer output signal according to thethreshold voltage, wherein an operating voltage of a device connected tothe semiconductor device is detected based on the buffer output signalof each input buffer, and the voltage selection signal is generatedbased on the detected operating voltage.
 18. The semiconductor deviceaccording to claim 10, further comprising: a first input buffer having afirst low-potential power supply terminal, connected to a firsthigh-potential power supply, and a first high-potential power supplyterminal, connected to a second high-potential power supply having avoltage higher than a voltage of the first high-potential power supply,wherein the first input buffer receives an external signal and generatesa buffer output signal; and a second input buffer having a secondlow-potential power supply terminal, connected to a low-potential powersupply, and a second high-potential power supply terminal, connected tothe second high-potential power supply, wherein the second input bufferreceives the buffer output signal from the first input buffer andgenerates the voltage selection signal.
 19. A semiconductor device,comprising: an internal circuit for generating an output signal; and anoutput circuit, connected to the internal circuit, for converting avoltage level of the output signal of the internal circuit, the outputcircuit including: a plurality of first output transistors connected toa plurality of high-potential power supplies having different voltages,respectively; a second output transistor connected to a low-potentialpower supply and to the plurality of first output transistors; and acontrol circuit for selecting, in a first mode, one of the plurality offirst output transistors according to a voltage selection signal andcausing the selected first output transistor and the second outputtransistor to perform a level conversion operation, and for sequentiallycontrolling, in a second mode, the plurality of first output transistorsaccording to the voltage of each of the plurality of high-potentialpower supplies.
 20. The semiconductor device according to claim 19,wherein the plurality of first output transistors include: a firsttransistor connected to a first high-potential power supply; and asecond transistor connected to a second high-potential power supplyhaving a voltage higher than a voltage of the first high-potential powersupply; wherein the second output transistor is connected between thelow-potential power supply, the first transistor, and the secondtransistor; and the control circuit first activates the firsttransistor, and then activates the second transistor.
 21. Thesemiconductor device according to claim 19, further comprising: a signalgeneration circuit, connected to the output circuit, for generating thevoltage selection signal, wherein the signal generation circuit includesa fuse that is broken when a voltage selection signal for selecting aspecific one of the plurality of output cells is generated.
 22. Thesemiconductor device according to claim 19, further comprising: arewritable storage unit for storing setting of voltage; and a signalgeneration circuit, connected to the storage unit, for generating thevoltage selection signal based on the stored setting information. 23.The semiconductor device according to claim 19, further comprising aplurality of input buffers, each having a different threshold voltageand generating a buffer output signal according to the thresholdvoltage, wherein an operating voltage of a device connected to thesemiconductor device is detected based on the buffer output signal ofeach input buffer, and the voltage selection signal is generated basedon the detected operating voltage.
 24. The semiconductor deviceaccording to claim 19, further comprising: a first input buffer having afirst low-potential power supply terminal, connected to a firsthigh-potential power supply, and a first high-potential power supplyterminal, connected to a second high-potential power supply having avoltage higher than a voltage of the first high-potential power supply,wherein the first input buffer receives an external signal and generatesa buffer output signal; and a second input buffer having a secondlow-potential power supply terminal, connected to a low-potential powersupply, and a second high-potential power supply terminal, connected tothe second high-potential power supply, wherein the second input bufferreceives the buffer output signal from the first input buffer andgenerates the voltage selection signal.
 25. A semiconductor devicecomprising: an internal circuit for generating an output signal; and anoutput circuit, connected to the internal circuit, for converting avoltage level of the output signal of the internal circuit, wherein theoutput circuit includes a plurality of output cells generating aplurality of level-converted output signals having different voltages;and a plurality of pads respectively connected to the plurality ofoutput cells.